As a promising switching circuit with a field-effect transistor (hereinafter referred to as `FET`) for extreme high frequency band, a semiconductor device in which an inductor is connected in parallel between the source and drain of FET is proposed (Iyama et al., "Inductor Built-in FET Switch", Technical Report of IEICE, Vol. MW-96-71, pp.21-26, July, 1996)
FIG. 1 is a circuit diagram showing a conventional switching circuit. In FIG. 1, an inductor 123 is connected in parallel between the source and drain of FET 121, and a switching is conducted between a first terminal 125 and a second terminal 126 when FET 121 is turned on/off. Though FET 121 is a three-terminal element, FET 121 can be equivalently represented as a two-terminal element because the bias line connected with the gate is opened in RF manner when a sufficient large resistance 124 is connected to the gate. Namely, FET 121 is equivalent to a capacitance C when it is turned off, and it is equivalent to a resistance R when it is turned on.
FIG. 2 is a circuit diagram showing the equivalent circuit that FET in FIG. 1 is turned off, and FIG. 3 is a circuit diagram showing the equivalent circuit that FET in FIG. 1 is turned on.
As shown in FIG. 2, when FET is turned off by applying a voltage lower than the pinch-off voltage, the circuit between the first terminal 125 and the second terminal 126 becomes equivalent to a circuit that the capacitance C and the inductor L are connected in parallel. In this case, isolation Is between the first terminal 125 and the second terminal 126 is given by: ##EQU1##
Here, a resonance frequency f.sub.0 for the parallel-connected capacitance C and inductor L is given by: ##EQU2##
When a signal with the resonance frequency f.sub.0 input, electric power to be transmitted from the first terminal 125 to the second terminal 126 becomes zero. In this case, isolation Is becomes ideally infinite.
However, even when the frequency of a signal input to the first terminal 125 is slightly deviated from the resonance frequency, isolation Is is highly reduced. For example, in the conventional semiconductor device in FIG. 1, isolation Is is 10 dB at the resonance frequency f.sub.0 =37 GHz. But, when the frequency becomes 35 GHz, isolation Is is reduced to 7 dB.
On the other hand, when FET is turned on as shown in FIG. 3, the circuit between the first terminal 125 and the second terminal 126 becomes equivalent to a circuit that the resistance R and the inductor L are connected in parallel. In this case, electric power to be transmitted from the first terminal 125 to the second terminal 126 is given by; ##EQU3## where the impedances of the first terminal 125 and the second terminal 126 are Z.sub.0. In this case, according as the frequency f is increased, insertion loss IL goes, from zero, near to: ##EQU4## The insertion loss of the conventional semiconductor device in FIG. 1 is 1.3 dB at 37 GHz.
Meanwhile, in the conventional switching circuit, the ideal values of insertion loss and isolation Is for. e.g., a signal of 94 GHz can be calculated using expressions [1] and [3]. FIG. 4 shows the calculation results. In FIG. 4, the resonance frequency f.sub.0 is 92 GHz for L=100 pH and C=0.03 pF. A label "ON" in FIG. 4 indicates frequency characteristics of an ON (turn-on or closed) state of the switch. A label "OFF" indicates frequency characteristics of an OFF (turn-off or opened) state characteristics of the switch. Herein, a frequency range with isolation Is greater than 20 dB is defined as `effective band`. Thus, the effective band of the switching circuit in FIG. 1 becomes 5.3 GHz.
Accordingly, in the conventional switching circuit, there is a problem that the effective band is thus narrow.